1. Field of the Invention
Embodiments of the present invention relate to a connector with a ball joint operable as a bending relief joint, to relieve the bending stress applied at high bending locations in a subsea riser, in the event that the applied bending moments exceed a predetermined bending limit.
2. Description of the Prior Art
Offshore oil and gas production installations typically have a wellhead on the seabed and production connections on a subsea tree. For production completion or workover purposes, a riser pipe string may be connected between the subsea tree and a surface vessel, such as a semi-submersible or drill-ship responsible for carrying out the desired well intervention.
The surface vessel may be maintained in position above the well by means of anchors or by dynamic location using the global positioning system (GPS) and thrusters in the vessel to counteract drift. In operating conditions, some angular displacement of the surface vessel away from the vertical above the well will be acceptable to cope with drift caused by changes in local conditions, including tides and weather. In a given situation the operating range within which a surface vessel may safely drift will depend on various factors, including the tensile and bending limits of the riser. In unexpected or extreme circumstances, for example if an anchor slips or GPS fails or in a sudden storm, the surface vessel may drift to the extent that emergency action may be required to avoid damage to the riser or the wellhead.
For such purposes, the riser string may include a safety joint—a ‘weak link’ designed to fail in tension below the limits of an ordinary riser pipe. To protect the wellhead, an emergency disconnect package (EDP) utilizing a quick disconnect connector may be provided at the base of the riser string.
In the past, various types of ball joint or flexible connectors have been proposed to allow some degree of continuing flexure in a tubular riser string.
However, these systems are not themselves designed to be triggered by, or to provide protection against, excessive bending moments which may be applied to or by the riser, in cases of extreme angular displacement from the vertical. Further difficulties may also arise owing to local environmental differences such as strength of tides and depth of the water at the well site. In shallower water, a surface vessel is necessarily more limited in absolute terms in the extent of allowable drift, and the shorter length of riser behaves more rigidly than longer lengths in deep water. Different stresses arise and safety systems need to be adapted correspondingly.